The present invention relates to electric machines, such as motors or generators, and more particularly to external rotor, brushless permanent magnet (BPM) electric machines having aluminum windings.
Electric machines, such as motors or generators, typically include a stator mounted inside a housing and a rotor supported in the housing for rotation relative to the stator. The stator and/or the rotor of the machines have windings comprising coils of wire. In motors, electricity is delivered through these coils to generate magnetic fields, causing the rotor to turn. In generators, the electricity is generated in the windings as the rotor turns. Although different electrically conductive materials may be used to make the windings, in the past these windings have frequently been made from copper or copper alloys. Electric machines are often integrated into appliances such as washing machines and apparatus such as stationary bicycles and treadmills. The size, performance and cost of the appliance incorporating the electric machine may be important factors when making purchasing decisions. The size, performance and cost of the electric machine frequently have a significant impact on the overall size and cost of the appliance.
Conventional high-torque, low speed motors with copper windings often have shorter teeth, resulting in reduced (i) noise, (ii) ampere drops in the teeth, and (iii) deleterious flux saturation effects. More specifically, shorter teeth are structurally stronger to resist forces, and reduce vibration and noise. Also conventionally, some short teeth were too thin and structurally weak to resist vibration, requiring other mechanical means to strengthen the teeth, such as additional structural members connecting the free ends of adjacent teeth to one another (e.g., wires). Moreover, shorter teeth, which carry flux, require less ampere turns (i.e., current) to drive the flux through the teeth and reduce the deleterious effects of flux saturation. Thus, lower driving currents result in lower wire current (i.e., I2R) losses. Moreover, excessive saturation can result in non-linear torque per amperage at high torque loads. Thus, in the past, to optimize motor design, copper windings were paired with desirable short teeth to optimize performance of the motor and reduce these unwanted effects.
Further, in the case of washing machines, motor weight can affect washing machine performance because higher motor weights increase susceptibility of the washing machine to unbalance. Thus, lighter motors are desirable. Conventionally, copper windings are used in motors for washing machines. If other conductive materials having lower density, such as aluminum, were used, washing machines that are less likely to become unbalanced could be made. But most of these lower density materials are less conductive than copper. Therefore, larger gauge wire must be used in the windings. The larger wire gauge allows greater heat transfer, potentially resulting in lower motor operating temperatures.
In addition, prices of conductive materials fluctuate. At times, some winding materials are more expensive than others. For example, sometimes copper is very expensive relative to other electrically conductive materials such as aluminum. Electrical conductivity of the material is also a significant consideration. As conductivity decreases, larger windings must be used to overcome losses. However, larger windings raise concerns about machine size and resulting appliance size. Thus, even though some materials such as aluminum can have a cost advantage over more frequently used materials such as copper, copper may still be used due to these other concerns such as size. Conventionally, modifying a motor designed for copper windings to accept aluminum windings at similar performance required a motor having a greater volume. This volume increase was often accomplished by increasing the diameter and/or the length of the motor to accommodate increased aluminum winding volume over the conventional copper winding volume. But if the exterior size of the motor is restricted by the application in which the motor is to be used, these increases in length and/or diameter cannot be accomplished and the alternative winding material (e.g., aluminum) cannot be used without sacrificing performance. It would be advantageous if the electric machine could incorporate more cost effective materials (e.g., aluminum windings) while maintaining similar performance in a motor having a substantially equal diameter and length.